1. Field of the Invention
The present invention relates generally to tubing assemblies. More particularly, the present invention relates to a method for brazing aluminum tube assemblies in a controlled atmosphere furnace.
2. Disclosure Information
Historically, the most common method of brazing aluminum A/C fittings into tubing has been the open-air flame braze rotary table process. As is known in the industry, this type of process involves a rotating table onto which various tube assemblies are placed. As the table slowly rotates, the assemblies pass from one workstation to the next. Each table is manned by personnel who perform the operations of loading and unloading the assemblies onto the table. Although this process is used widely in the industry and provides adequate results, it is a highly specialized process and contains many complexities. The complexities arise from decisions addressing issues such as, whether to use filler metal in wire form, preformed rings or paste, or whether to choose a corrosive or non-corrosive fluxing agent. In addition, the rotary table provides many other problems that have to be controlled. Precise design of the table has to take into account issues that arise from adequate flame temperature, gas/air ratios, level of cooling (water, air), maintaining gas and air flow rates, air drafts (within a plant that can disturb flame patterns), and the development of the flame patterns. These are only a few of the attributes that need particular attention to insure a robust and capable process. Lastly, much of the success of a rotary braze table process depends on the part fixturing and maintaining the relationship of the part to the flame. Due to the design complexity of A/C tube assemblies many of the tables/fixtures are capable of brazing only one or two joints per table. Subsequently, when a new joint requires brazing, many times a new table and fixtures will be purchased. This occurs because of the vast differences in part design and the precision required to maintain part orientation to the flame.
Currently, tube assemblies made from steel alloys are successfully brazed in controlled atmosphere brazing ("CAB") furnaces. For example, U.S. Pat. No. 4,294,395 discloses a process for brazing ferrous parts in a copper brazing furnace. Heretofore, successful CAB brazing of aluminum tube assemblies, either at a tube-to-component joint or a tube-to-tube joint have not proven successful. Two primary difficulties arising from furnace brazing of aluminum tube assemblies are the need for self-fixturing the components and the different heat absorption/transfer characteristics of the aluminum components to be furnace brazed. To realize the best economic efficiencies of furnace brazing, the components should be self-fixturing, meaning that many parts can be brazed on a moving conveyor into the furnace with no additional mechanical fixturing needed. Aluminum tube assemblies have historically been flame brazed because fixturing was required due to the unique configuration of the assemblies.
Furthermore, as is well know, aluminum is an excellent heat conductive material. Aluminum tube assemblies, such as those used in automotive or commercial A/C systems includes various components of different compositions, sizes and masses. Because of these differences, the components heat up at different rates. For example, an aluminum manifold block, used for joining one aluminum tube to another, may weigh as much as twice a different component. As such, it takes longer for the heavier component to get to braze temperature. However, this presents a problem in that once the heavier component is at temperature for the brazing material or paste to flow, the braze paste used in connecting other, lighter components has already flowed and most likely, has been at temperature for too long, causing either a degradation in the paste, or complete run-off of the paste from the assembly. Therefore, it would be advantageous to propose a process which solves both of these issues.
It is an object of the present invention to provide a process for brazing aluminum tube assemblies in a controlled atmosphere brazing furnace without fixturing the components to the moving furnace conveyor.
It is a further object of the present invention to provide a method for brazing aluminum tube assemblies in a furnace, with the assembly having components of various sizes and masses.